A platinum group-based alloy is designed using heat resistance, oxidation resistance, and chemical resistance of a platinum group metal, and is widely used as a high-temperature member or a corrosion-resistant product. The platinum group metal as used herein collectively refers to Pt, Pd, Rh, Ir, Ru, and Os.
Processes for production of a platinum group-based alloy generally include a compounding step, a melting step, a plastic working step, and the like for an alloy raw material. The melting method can be classified into several types. A platinum group metal, which is a main component of the platinum group-based alloy, has a very high melting point, and hence an induction heating melting furnace or an energy beam melting furnace is used.
The mainstream of induction heating melting has been a melting method involving using an oxide-based refractory crucible in a vacuum or an inert gas, while a cold crucible has recently been tried out (for example, Patent Literature 1).
Non-consumable electrode-type arc melting, consumable electrode-type arc melting, vacuum plasma melting, electron beam melting, and the like have been applied to energy beam melting, and the mainstream is the non-consumable electrode-type arc melting (for example, Patent Literature 2). The non-consumable electrode-type arc melting is a method involving forming an arc column between a W (tungsten) electrode having a sharply polished discharge end and an alloy raw material placed on a boat-shaped water cooled copper crucible, and melting the alloy raw material through use of the arc column as a heat source. The consumable electrode-type arc melting is a melting method involving using, as an electrode, a raw material itself, and forming an arc column between an end of the electrode and a water cooled copper crucible. By virtue of a melting ability of several hundred kilograms, the consumable electrode-type arc melting is used for production of a non-noble metal, such as Ti, but is not used for melting of the platinum group-based alloy. The vacuum plasma melting and the electron beam melting have a refining action because of melting in a vacuum or a high vacuum, and in addition, are suited for mass melting because a beam having a high energy density is used (for example, Patent Literature 3).
In the induction heating melting, a molten ingot is generally produced by melting an alloy raw material in a refractory crucible and inclining the crucible to pour and cast the alloy raw material into a casting mold. The refractory crucible has a temperature limit, and is used for production of a platinum group-based alloy having a relatively low melting point (roughly 2,000° C. or less). This method has an advantage of being capable of producing several ten kilograms of the molten ingot in a short time, but entails a risk of inclusion of refractory owing to inevitable contact between the refractory crucible and a molten metal, sometimes resulting in mixing of the refractory in the molten ingot. In addition, this method also generates casting defects, such as a shrinkage cavity, pores, and casting surface roughness, and has a problem of a low material yield owing to the need for removal processing of defect portions, such as cutting, trimming, or grinding.
In the non-consumable electrode-type arc melting, the discharge end of the W electrode gradually wears in a long melting time (arcing time), and melting cannot be continued owing to stop or wandering of the arc column. Therefore, a melting operation needs to be interrupted and the discharge end of the W electrode needs to be re-polished. In addition, in combination with a relatively small irradiation area of the arc column, continuous casting cannot be performed. That is, productivity is poor, and the amount of the alloy meltable at one time is limited to about several kilograms. In addition, it is general to reduce pressure to less than 0.8 atm during melting. When an alloy containing component elements having largely different vapor pressures is melted, a component element having a higher vapor pressure vaporizes more and an alloy composition varies.
The vacuum plasma melting and the electron beam melting generally have the ability to continuously cast an alloy in a large amount as compared to the non-consumable electrode-type arc melting, and are suitable for melting of a pure metal because impurities can be vaporized and removed (refining effect) by virtue of a vacuum melting atmosphere. However, in melting of the alloy, a component element having a higher vapor pressure vaporizes more and an alloy composition varies.
As described above, the melting methods, which have hitherto been widely used, have their limits in producing the platinum group-based alloy without a compositional variation in a large amount at high yield.